This just started to happen to me on my new 27" iMac after upgrading to 10.7.4
I can ssh into the iMac from my other mac and 'top' tells me that the screensaver process is 'stuck'.
It's very intermittent though, sometimes it all works fine for days but sometimes it sticks every time the screensaver (doesn't seem to matter which one I choose) activates.
See also https://discussions.apple.com/thread/3949663?answerId=18398072022#18398072022
If I SSH into the iMac and attempt to kill the screensaver from the command line by executing
ps ax | grep -i screensaver | grep -v grep
to get the process ID, then
kill -HUP {whatever the process ID is}
the screensaver process is killed okay but the screensaver image stays frozen on the screen.
(see http://blog.tel-cor.com/2009/03/03/unlocking-the-os-x-screen-saver-from-the-comm and-line/)
The only other 'stuck' process now according to 'top' is 'VTDecoderXPC'
There is some discussion of this at https://plus.google.com/117666625199895400127/posts/fXMwnH6DzSj where they indicate that it's an HTML5 / Safari extension thing. I do have a few Safari extensions that relate to HTML5 as I uninstalled Flash a while ago.
I have found the cleanest way to get control of my mac back is, unfortunately, to ssh in and
sudo shutdown -r now
which reboots the mac. That occasionally has the side-effect that it forces Spotlight to want to reindex my hard drives, which is rather time consuming.
If anyone can suggest a better way to recover from this I'm all ears.
In the meantime I'll keep experimenting and post any fixes or workarounds here.
